Modifying the Mass Air Flow Sensor

This page dicusses how to calibrate your MAF when you've upgraded to larger fuel injectors. My stock 2.3L engine came with 14 lb/hr fuel injectors, I've upgrade to 19 lb/hr injectors in the quest for more power. Basically, you drill out the MAF sample tube to a larger size that is proportional to the increase in size of the new injectors.

It worked very well and overall driveability is great! The first thing that I noticed was that on starting from a stop, I didn't have the little bog that I had before modifying the MAF. In 5th gear, I can cruise at 1400 rpm going 70 km/h (43.5 mph) and accelerate. This is a good mod!

I have a steep long test hill that I use to compare the performance of modifications and ran it first without the mod and then after with the mod (within 1/2 hour of each run). All runs were started at the same speed at the bottom of the hill (100 km/h - 62 mph) in 5th gear. The first run without the mod, with my foot to the floor, I was only able to reach 95 km/h (59 mph) in 5th gear at the top of the hill. With the mod, I was able to go 105 km/h (65 mph) at the top - I accelerated up the hill! An increase of 10 km/h - that is impressive and one of the best improvements yet.

NOTE: Do NOT modify the original MAF, pick up a used one from the wreckers, verify that it works in its stock configuration and modify it. This way, you can easily go back to the original MAF and have a running vehicle.

Drill out the sample tube to 7/16"

First you must remove the MAF sensor. It is held on the top with two security Torx screws. If you have a security Torx screwdriver then you are ready to go. If not, you may be able to break off the center pin with another small screwdriver - this was very hard for me to do but it is possible with a little bit of swearing.

It is also a good time to clean the MAF elements. They are the small globs held by micro thin wires in the airflow. Use a spray contact cleaner or a very small brush and alcohol. They are very delicate so don't go postal on them!

Drill the MAF sample tube.

Basically, the idea is that you are recalibrating the air/fuel curve for larger injectors. So for an increase from 14 lb to 19 lb (19/14 => 135.7% increase), you have to increase the cross sectional area of the MAF injector sample tube by 35.7%. In other words drill it bigger proportionally. For the 14 to 19 lb injector increase, you would drill the original sample tube from 25/64" to ideally 29/64" (this corresponds to a 35.7% increase in cross sectional area).

I used a smaller 7/16" drill because I was just handholding the piece in my drill press and letting the drill self center - the smaller drill should compensate for any drilling errors and the ECU will compensate for the small size difference. The MAF sample tube is aluminum and easily drilled. You don't have to remove the sample tube support tube backing plate as there is a space before the drill hits it.

Questionable Modification: Removing the MAF "support"

Here's a MAF mod that has gone bad. The person cutout the center support tube:

Stock MAF

Modified 2.3L MAF looks good BUT it the wrong thing to do!

The center tube looks like it is just a support tube but it has a purpose. If you look closely at the unmodified MAF, you'll see that the sample tube doesn't go straight through, it flows down the support tube and exits at the bottom. There is a plate covering the air flow tube. This tube is calibrated in length and by sampling from the top side of the MAF housing and exiting on the bottom. The bottom of the support tube will actually aid in drawing air through the tube. It gets a more accurate flow reading. If you have a bend in your air flow just before the MAF, you can get all the air flowing along one side of the MAF housing and missing the sample tube with this modified setup!

A Better Support Tube Mod

I think that a better mod would be to block off the complete support tube rather than leaving it open at the bottom and drill a row of holes in the support tube plate whose total area would match the holesize in the sample tube. I would go even further and drill the holes smaller starting at the sample tube to larger at the bottom. Or modify the original plate by adding one hole directly behind the sample tube that is the same size as the difference between the new sample tube size and the original sample tube size. In the worst case, I'll have made a whistle! I will definitely be trying this as you can mount the plate with two screws and swapping out different plates for testing would be trivial.

Here's a long read on what exactly is MAF calibration! And if you haven't read enough, here's one on the theory behind modifying the MAF.

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Copyright February 2011 Eugene Blanchard